Bimodal Molecular Weight Distribution Formed in Emulsion Polymerization with Long‐Chain Branching

Abstract

It is known that the molecular weight distribution (MWD) formed in emulsion polymerization of ethylene can be bimodal. A simplified model is used to investigate the emulsion polymerization that involves chain transfer to polymer, aiming at finding necessary conditions to form a bimodal MWD. According to the present theoretical investigation, a bimodal MWD can be formed when the probability that the primary chain end is connected to a backbone chain, P b is larger than 0.5. The bimodality for these cases results from the limited volume effect, that is, the high molecular weight profiles are distorted by the small particle size, which is comparable to the size of the largest branched polymer molecule formed in a particle. During Interval II, the P b ‐value could be approximately equal to C p x c /[C p x c +C m (1 −x c )] in usual emulsion polymerization without using the chain transfer agents, where C p and C m are transfer constants to polymer and to monomer, respectively, and x c is the conversion at which Interval II ends, and therefore, one can predict the possibility of obtaining a bimodal MWD on the basis of these reaction parameters. On the other hand, if the experimentally obtained MWDs are bimodal even when P b < 0.5, the origin of bimodality would be attributed to other reaction mechanisms, such as the chain‐length dependent branching reactions and combination of two different MWDs formed in large and small polymer particles.